Dr. Lightner Witmer was born in 1867, two years after the end of the American Civil War, in Philadelphia, Pennsylvania to a devout Catholic mother and father. He obtained his A.B. from the University of Pennsylvania in 1888. After teaching briefly at a secondary school and flirting with the possibility of a career in law, he entered the graduate psychology program at the University of Pennsylvania under James McKeen Cattell. Later, when Cattell moved to Columbia University in New York, Witmer transferred to Leipzig, Germany, and earned his doctorate under Wilhelm Wundt.

After completing his work in Germany, Witmer returned to the University of Pennsylvania and took over Cattell's laboratory. Although Witmer resumed his experimental research, his interests gradually began to focus on the theme of making scientific psychology "practical" ( Witmer, 1896 , 1897), and in 1896 he established the world's first psychological clinic. However, his commitment to laboratory research continued for some time; thus, in 1902 he published a laboratory manual, and in 1904 he collaborated with E. B. Titchener to form the group later known as the Society of Experimental Psychologists.

In 1908, believing that it was sometimes necessary, in order to bring about successful behavioral change in a child, to have overall environmental control for a period, he established and staffed a small private residential school near Wallingford, Pennsylvania. Later, he established a similar, but larger facility in Devon, Pennsylvania.

Both Witmer and G. Stanley Hall (1844-1924),who founded the American Psychological Association (APA) in 1892, were important in the early development of what is now School Psychology. Witmer used an idiographic clinical model, while Hall used a nomothetic approach. Idiographic has been defined as "attempts to understand a particular event or individual", and a nomothetic model is defined as "characterizing procedures and methods designed to discover general laws" (Fagan & Wise, 1994, p.28). Their students combined their concepts and practices, which influenced the acceptance of school psychological services (Fagan & Wise, 1994).

Lightner Witmer was married to Emma Repplier Witmer who was also a Catholic.